The present embodiments relate to operating a magnetic resonance system.
In magnetic resonance tomography, various magnetic fields are provided (e.g., a base field, gradient fields and radiofrequency pulses).
Using the static base field (e.g., the B0 field), the spins of the scanned object are aligned in the direction of the base field. By the emission of a radiofrequency excitation pulse (MHz range) (e.g., the B1 field), the spins are rotated (e.g., “tilted”) out of the direction of the base field into another direction. After the end of the radiofrequency excitation pulse, the nuclear spins return into the direction of the B0 field. This process induces an electrical signal in the receiver coils, which is then used for the image calculation.
The gradient fields lie in the low-frequency range (kHz range) and are locally superimposed on the base field. A separate term is not generally used for the gradient fields.
In order to be able to deflect the spins, the frequency of the excitation pulses are to be adjusted accurately to the resonant frequency determined by the strength of the B0 field. The direction of the B1 field is to be orthogonal to the direction of the B0 field. The excitation pulses are to rotate around the base field with the resonant frequency. The rotation of the B1 field of the excitation pulses may take place either in the clockwise sense or in the counterclockwise sense. Only one of the two rotation directions (e.g., the rotation direction that coincides with the rotation direction of the nuclear spins around the B0 field) is effective for the deflection. This direction is denoted by the suffix “+”. The other is correspondingly denoted by the suffix “−”. With respect to the imaging, the B1+ component of the B1 field is thus referred to. For technical reasons, however, a pure B1+ field may be generated only with difficulty. For a small oil-filled sphere, this situation is generally quite sufficient, while the geometry and inhomogeneous structure of a patient almost always also leads to a more or less sizeable B1− component.
Various scanned objects do not react in entirely the same way to exactly the same excitation pulses. Before imaging measurement sequences with corresponding excitation pulses are started, the way in which the radiofrequency amplifier is to be driven so that the desired B1+ amplitudes are generated in the scanned object is to be ascertain. This is done by “transmitter adjustment”, which may be interpreted as an “amplifier adjustment procedure”. This procedure is carried out as follows.
Beginning with an appropriate (e.g., approximately correct) starting amplitude based on empirical values, a particular number of radiofrequency excitation pulses are transmitted. The amplitude profile and time spacing of the radiofrequency excitation pulses are exactly defined. The magnetic resonance signal excited by the excitation pulses is determined in terms of the B1+ amplitude achieved. If this amplitude differs by more than a predetermined tolerance from a reference amplitude to be adjusted, the driving of the radiofrequency amplifier is then modified according to the difference ascertained. The measurement procedure described above is then repeated until the amplitude obtained lies in the acceptable tolerance range. In general, a few such iterations are provided in order to ascertain the correct driving of the radiofrequency amplifier for a given situation (e.g., scanned object, transmitter coil and position of the scanned object relative to the transmitter coil).
By the emission of the radiofrequency excitation pulse, power is developed in the hardware of the transmission path (e.g., including the transmitter coil), but also in the scanned object. This power is to be determined and monitored by measurement technology. The monitoring is provided in order to protect the hardware itself against arcing and overheating. The monitoring is provided in order to protect the patient against burns and overloading of the circulation. Magnetic resonance systems are equipped with corresponding measuring apparatuses. The measuring apparatuses provide safe operation of the magnetic resonance system at any time. The measuring apparatuses are configured so that the measuring apparatuses are intrinsically safe. This provides that a single error cannot lead to failure of the measuring apparatuses as a whole. The measuring apparatuses are to be checked at sufficiently short time intervals (e.g., once a month) in order to provide that the measuring apparatuses are still operating correctly.
In order to provide correct operation of the respective measuring apparatus, various measures may be provided. For example, a technician may check the measuring apparatus at sufficiently short time intervals on site by using a suitable external measuring device, and/or the measuring apparatus, even if still functioning properly, may be constantly replaced with a new, correctly operating measuring apparatus. This procedure entails personnel costs and material costs.
As an alternative, a hardware-based solution may be provided. In this case, the measuring apparatus includes a plurality of subdevices that independently of one another monitor the radiofrequency power delivered to the scanned object, and also monitor one another. This procedure makes the measuring apparatus more complicated and more expensive.
One particular critical error is a slow drift of the measuring apparatus (e.g., not full failure of the measuring apparatus but gradual loss of the calibration). In the case of a plurality of measuring apparatuses, all the measuring apparatuses are not to drift simultaneously in the same direction.